The solid state image pick-up device used as an image pick-up device for portable video cameras, electronic still cameras and image inputting devices is smaller and lighter compared with the conventional iconoscope, and therefore, its field of application is being gradually expanded, because the apparatuses adopting the solid state image pick-up device can be miniaturized.
A CCD (charge coupled device) is one such solid state image pick-up device. In this type of device, the photo-accumulation effect of the CCD is utilized. The constitution of this device is schematically illustrated in FIG. 1.
The illustrated image pick-up device 1 is constituted such that photo converting sections 7 consisting of photo diodes and photo transistors are arranged on a board in rows and columns and in the pattern of (400-500).times.(250-300). The device also includes vertical transfer sections 8 for vertically transferring charges generated through the conversion of light rays received by the photo converting sections 7, from these sections to horizontal transfer sections 9. Sections 9 output these charges in the form of output signals 10 or drain charges 11 after receipt thereof from the vertical transfer sections 8.
The vertical transfer sections 8 and the horizontal transfer sections 9 respectively consist of the CCD devices, and are also provided on their surfaces with a plurality of transmission electrodes which are for transmitting the charges in accordance with driving pulses supplied from a driving pulse generating circuit (to be described later).
A video camera adopting such an image pick-up device 1 is illustrated in FIG. 2.
This video camera comprises: an optical system 3 consisting of lenses and prisms for focusing and lenses and prisms or a mirror for spectroscopically breaking down incident light rays 2; an image pick-up device, e.g., the CCD type solid state image pick-up device as illustrated in FIG. 1, for converting the light rays into output signals after receipt of the light rays which have been controlled by the optical system, and for supplying the output signals to a video circuit (not shown); and a driving pulse generating circuit 16 for driving the image pick-up device 1 in the required periods.
In the case of video cameras, an image quantity equivalent to about 60 frames per minute, is required and therefore, the image pick-up device 1 will have a field period of 1/60 seconds. If a clear image is to be obtained when picking up a speedy object, only the signal charges accumulated during one field period, i.e., during 1/250-1/2000 seconds are outputted, while the signal charges accumulated during the periods other than the above mentioned period become needless and have to be drained away in the form of drain signals. This signal charge accumulation time can be compared to the shutter period of a camera.
Meanwhile, the solid state image pick-up device, e.g., the CCD, is very sensitive to the amount of the incident light rays, and therefore, the resolving power becomes insufficient upon encountering the case of an insufficient amount of light rays, while it produces the so-called blooming phenomenon upon encountering the case of an excessive amount of light rays, thereby degrading the image quality. Therefore, the need arises for properly adjusting the amount of light rays incoming into the image pick-up device.
The conventional image pick-up apparatus is of the "fixed shutter time" type, in which the amount of incident light rays 2 incoming through an iris mechanism into the image pick-up device 1 is adjusted in a state with the above described signal charge accumulating time, i.e., a fixed shutter time.
That is, according to the conventional image pick-up apparatus as shown in FIG. 2, a part of the output signals from the image pick-up device 1 is received in a signal level detecting circuit 14 for detecting the level of the signals, and an auto iris circuit 15 which controls an iris driving section 17 consisting of a motor or a solenoid in such a manner that the average level of the output signals should be always constant. In turn, the iris driving section 17 drives an iris mechanism 18 in such a manner that the amount of incident light rays incoming into the image pick-up device 1, is adjusted.
The light amount adjusting means used in such a conventional image pick-up apparatus is constituted such that a mechanical type iris adjusting mechanism is used in a state with the shutter time is fixed, and therefore, its constitution is very complicated, thereby raising the manufacturing cost, and making miniaturization difficult. Further, in the case where the object is moving at a high speed, the response speed becomes very slow, while if the brightness of the object is too high or too low, its discrimination is almost impossible.
Further, this mechanical light amount adjusting means consumes large amounts of power, and therefore, portable video cameras or electronic still cameras using batteries as the power source have difficulties because of the speedy depletion of the battery source.